1. Field of the Invention
The present invention relates to a semiconductor device which has a circuit composed of a thin film transistor (hereinafter referred to as a TFT) and a manufacturing method thereof. The present invention relates to, for example, an electro-optical device represented by a liquid crystal display panel and electronic equipment in which such an electro-optical device is mounted as a part.
Note that in this specification, a semiconductor device indicates a general device which functions by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are each a semiconductor device.
2. Description of the Related Art
In recent years, the development of a semiconductor device, which has a large area integrated circuit manufactured from a thin film transistor (TFT) using a semiconductor thin film (about several nm to several hundred nm in thickness) formed on a substrate with an insulating surface, has been progressed. An active matrix liquid crystal display device, an EL display device, and a contact type image sensor have been known as typical examples thereof. In particular, since a TFT using a crystalline silicon film (typically, a polysilicon film) as an active layer (hereinafter referred to as a polysilicon TFT) has high field effect mobility, it is also possible to form various functional circuits.
For example, in the case of the active matrix liquid crystal display device, a pixel circuit for displaying an image in each functional block and a driver circuit for controlling the pixel circuit, which is composed of a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like using CMOS circuits as fundamental circuits, are formed on a single substrate.
In the case of the pixel circuit of the active matrix liquid crystal display device, TFTs (pixel TFTs) are arranged in several tens to several millions of pixels and a pixel electrode is provided in each of the pixel TFTs. An opposing electrode is provided at an opposing substrate side sandwiching liquid crystal, and a kind of capacitor using the liquid crystal as a dielectric is formed. Thus, voltage applied to each of the pixels is controlled by a switching function of the TFT to control a charge to the capacitor and drive the liquid crystal, the amount of transmitting light is controlled to display an image.
The use of such an active matrix liquid crystal display device has been diversified and the demand for high definition, high aperture ratio, and high reliability has been increased as a screen size becomes larger in area. Also, simultaneously, the demand for an improvement of productivity and a reduction in a cost has been increased.
An advantage of the active matrix display device is that an integrated circuit such as a shift register, a latch, or a buffer, which is composed of TFTs, can be formed as a driver circuit for transmitting signals to a pixel portion on the same substrate. Thus, the number of contact points with external circuits can be extremely reduced to improve the reliability of the display device.
Also, the pixel TFT is an n-channel TFT and used as a switching element for applying voltage to liquid crystal for driving. Since the liquid crystal is driven by an alternating current, a method called frame inverting drive is employed in many cases. In this method, in order to decrease the power consumption, it is important to sufficiently reduce an off current value (drain current flowing in placing the TFT in an off operation state) as a characteristic required for the pixel TFT. In addition, a characteristic such as a sufficient small parasitic capacitance between a gate and a drain is required. Since a pixel capacitance is small to cause an insufficient retaining operation, an auxiliary capacitor is provided in a pixel to implement such an operation and to prevent the influence of the parasitic capacitor.
A low concentration drain (LDD: lightly doped drain) structure has been known as a structure of a TFT for reducing an off current value. According to this structure, a region to which an impurity element is added at a low concentration is provided between a channel forming region and a source region or a drain region which is formed by adding an impurity element at a high concentration, and this region is called a LDD region.
Also, a multi-gate structure such as a double gate structure or a triple gate structure, which has a plurality of channel forming regions, has been known as a structure of a TFT for reducing a variation in an off current value. As shown in FIGS. 27A and 27B, when two TFTs are simply connected with each other to obtain a double gate structure, a total size of the TFTs for one pixel becomes large and this causes a reduction in an aperture ratio.
In a pixel for an active matrix driving system, a pixel electrode for applying voltage to liquid crystal is provided and a scan line (gate line) connected with a gate electrode and a data line connected with a source or a drain are intersected. Two types, an additional capacitor type in which the pixel electrode and the fore stage scan line (gate line) are overlapped with each other and a storage capacitor type in which a dedicated capacitor line is provided, have been known as auxiliary capacitors. In either of the two types, the allowable sizes of a TFT and an auxiliary capacitor for one pixel are necessarily reduced with an improved definition of an image. Thus, in order to obtain a high aperture ratio for respective pixels in a specific pixel size, it is essential that elements required for a structure of the pixels is efficiently laid out.
Also, with respect to the liquid crystal display device, two types, a direct view type for directly viewing an image displayed in a pixel portion and a projection type for displaying an image on a screen using an optical system, are being developed. Both types are selectively used based on a screen size, and the direct view type is used for a size up to about 30 inches and the projection type is used for the size or larger.
Also, in the case of all liquid crystal display devices, particularly, a liquid crystal display device for a projector, a variation in a characteristic of a TFT located in each pixel is caused by a photo leak current produced by incidence light into a semiconductor layer through various paths. Thus, a deterioration of an image quality (reduction in a contrast, a flicker, a cross talk, or the like) becomes a problem.